# std::filesystem::path::concat, std::filesystem::path::operator+=

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< cpp‎ | filesystem‎ | path

path& operator+=( const path& p ); | (1) | (since C++17)
path& operator+=( const string_type& str ); path& operator+=( std::basic_string_view<value_type> str ); | (2) | (since C++17)
path& operator+=( const value_type* ptr ); | (3) | (since C++17)
path& operator+=( value_type x ); | (4) | (since C++17)
template< class CharT > path& operator+=( CharT x ); | (5) | (since C++17)
template< class Source > path& operator+=( const Source& source ); | (6) | (since C++17)
template< class Source > path& concat( const Source& source ); | (7) | (since C++17)
template< class InputIt > path& concat( InputIt first, InputIt last ); | (8) | (since C++17)

Concatenates the current path and the argument

1-3,6,7) Appends path(p).native() to the pathname stored in *this in the native format. This directly manipulates the value of native() and may not be portable between operating systems.
4,5) Same as return *this += std::basic_string_view(&x, 1);.
8) Same as return *this += path(first, last);.

(6) and (7) participate in overload resolution only if `Source` and `path` are not the same type, and either:

- `Source` is a specialization of std::basic_string or std::basic_string_view, or
- std::iterator_traits<std::decay_t<Source>>::value_type is valid and denotes a possibly const-qualified encoding character type (char, char8_t, (since C++20)char16_t, char32_t, or wchar_t).

### Parameters

p | - | path to append
str | - | string or string view to append
ptr | - | pointer to the beginning of a null-terminated string to append
x | - | single character to append
source | - | std::basic_string, std::basic_string_view, null-terminated multicharacter string, or an input iterator pointing to a null-terminated multicharacter sequence, which represents a path name (either in portable or in native format)
first, last | - | pair of LegacyInputIterators that specify a multicharacter sequence that represents a path name
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- The value type of `InputIt` must be one of the encoded character types (char, wchar_t, char16_t and char32_t).
- `CharT` must be one of the encoded character types (char, wchar_t, char16_t and char32_t).

### Return value

*this

### Exceptions

May throw std::bad_alloc if memory allocation fails.

### Notes

Unlike with append() or operator/=, additional directory separators are never introduced.

### Example

Run this code

    #include <filesystem>
    #include <iostream>
    #include <string>

    int main()
    {
        std::filesystem::path p1; // an empty path
        p1 += "var"; // does not insert a separator
        std::cout << R"("" + "var" --> )" << p1 << '\n';
        p1 += "lib"; // does not insert a separator
        std::cout << R"("var" + "lib" --> )" << p1 << '\n';
        auto str = std::string{"1234567"};
        p1.concat(std::begin(str) + 3, std::end(str) - 1);
        std::cout << "p1.concat --> " << p1 << '\n';
    }

Output:

    "" + "var" --> "var"
    "var" + "lib" --> "varlib"
    p1.concat --> "varlib456"

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3055 | C++17 | the specification of concatenating a single character was ill-formed | made well-formed
LWG 3244 | C++17 | constraint that `Source` cannot be `path` was missing | added

### See also

appendoperator/= | appends elements to the path with a directory separator (public member function)
operator/ (C++17) | concatenates two paths with a directory separator (function)

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